Also in COPD epithelium, there is loss of the polymeric immunoglobulin receptor (pIgR)/secretory component system, which ensures the transcytosis and release of polymeric immunoglobulins into mucosal secretions, with impaired antiprotease defenses and post-translational epigenetic enzymes, such as histone deacetylase 2 (HDAC2), and accelerated cellular senescence (97, 98). The gene discussed is HDAC2; the disease is chronic obstructive pulmonary disease.